1. Field of the Invention
The present invention relates to a toner for use in developing an electrostatic latent image formed by a method such as electrophotography, electrostatic recording and electrostatic printing. In addition, the present invention also relates to a developer including the toner, and a developing device and an image forming apparatus using the toner.
2. Discussion of the Background
Electrophotographic image forming methods typically include the following steps:
(1) charging the surface of image bearing member such as photoreceptors with a charger utilizing discharging (charging step)
(2) irradiating the charged surface of the image bearing member with imagewise light to form an electrostatic latent image on the surface of the image bearing member (light irradiating step);
(3) developing the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member (developing step);
(4) transferring the toner image to a receiving material optionally via an intermediate transfer medium (transferring step); and
(5) fixing the toner image to the receiving material (fixing step).
In the fixing step, heat roller fixing methods in which a receiving material bearing a toner image thereon is sandwiched and fed by a pair of rollers including a heat roller are typically used because of having high energy efficiency. Recently, in view of environmental protection, a need exists for an image forming apparatus in which the energy consumption is minimized as much as possible. Therefore, it is necessary to reduce the heat capacity of the fixing members such as heat rollers and to develop a toner having good low temperature fixability.
However, toner having good low temperature fixability typically has a narrow fixable temperature range (i.e., poor hot offset resistance) and poor high temperature preservability. In attempting to remedy the drawbacks, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2001-305788 discloses a toner including plural binder resins having different molecular weights. Namely, the toner includes a resin having a relatively low molecular weight which is used to impart good low temperature fixability to the toner, and another resin having a relatively high molecular weight which is used to impart a good combination of hot offset resistance and high temperature preservability to the toner.
Polyester resins are typically used for imparting good low temperature fixability to toner. In conventional methods for producing polyester resins, organic tin (IV) compounds such as dibutyl tin oxide, titanium compounds such as tetra-n-butyltitanate, germanium compounds such as germanium oxide, manganese compounds such as manganese oxide, etc., have been used as catalysts. For example, it is described in JP-A 2001-305788 to use a polyester resin which is prepared using an organic tin (IV) compound as a catalyst. However, organic tin (IV) compounds, which have a Sn—C bond, are considered to be an endocrine disrupter. Therefore, it is not preferable to use-such organic tin (IV) compounds.
JP-A 2003-231744 discloses a toner including a polyester resin which is prepared using an inorganic tin (II) compound as a catalyst. It is described therein that by using such a polyester resin, the resultant toner has good charge rising property. However, JP-A 2003-231744 is silent on the low temperature fixability, hot offset resistance and high temperature preservability of the toner. As a result of the present inventors' experiment, it is found that the toner has drawbacks in that toner particles tend to agglomerate when environmental conditions (such as temperature and humidity) change and/or the toner particles receive a mechanical stress (for example, by being agitated in a developing device), resulting in formation of an undesired image in which agglomerated toner particles adhere to toner image portions and/or background portions of toner images (this problem is hereinafter referred to as agglomeration problem).
In order to prevent toner particle from agglomerating, JP-A 10-207112 (i.e., Japanese Patent No. 3407580) discloses a toner which includes, as external additives, a hydrophobized particulate inorganic material A having a number average particle diameter of from 5 to 70 nm and a particulate inorganic material B having a number average particle diameter of from 80 to 800 nm and including particles having a particle diameter not less than 1000 nm in an amount not greater than 20% by quantity. It is described in the publication that the particulate material A improves the fluidity of the toner and the particulate material B prevents the toner particles from adhering to the surface of the photoreceptor used.
Because of these reasons, a need exists for a toner which includes a polyester resin prepared using a safe material (an inorganic tin compound) as a catalyst and which can produce toner images without causing the agglomeration problem.